Weed Control That Makes Cents

September 25, 2015 09:56 PM

When cutting costs, prioritize proactive weed management

There was a time when Doug Winter wouldn’t have blinked at the site of a little waterhemp in his fields. After the steep rise in resistance, the southeastern Illinois corn and soybean farmer no longer sees waterhemp the same way. This behemoth of a weed steals yield and is resistant to glyphosate and five other herbicide groups.

“Waterhemp is probably our No. 1 concern, but marestail and giant ragweed also cause problems,” Winter says. “[We’re watching] the Palmer amaranth about 20 to 30 miles south.”

During the glory days of glyphosate, it was common to spray it as a pre-emergent and always plan a post application. That method might not hold true with today’s resistance. If you’re looking to reduce costs, the best way is to avoid post application altogether by using pre-emergent with residual.

Even as few as one plant for every 3' of row can reduce yields by up to 30%. If you have one weed per square yard this year, you could have 40 or 50 per square yard next year, says David Shaw, chair of the Weed Science Society of America herbicide resistance education committee. The amaranthus species produces nearly a million seeds, so even 90% control can be detrimental for the crop.

The biggest hurdle for many farmers with weeds is increased herbicide costs when resistance is isolated or not on their farm. When there appears to be no immediate benefit, many don’t want to change. 

Once resistance establishes a stronghold, it does not go away. It’s also easier to control a small seedbank than one that has taken over.

“In areas riddled with Palmer amaranth or waterhemp, we can easily spend $40 to $55 in herbicide costs per acre,” explains Bryan Young, associate professor of weed science at Purdue University. “Some farmers even pay $65 per acre for a rescue treatment and still don’t control all weeds. It’s how you use herbicide, not how much herbicide you buy.”

If you’re looking to cut herbicide costs, do not cut pre-emergent application as it brings the biggest return on investment. “We still have a lot of farmers in soybean weed management who don’t use as much residual as they should,” Young explains.

Effective weed control starts before weed and crop emergence. Give crops a fair start with pre-emergent herbicides and don’t let weeds grow taller than the crop.

“It seems like a lot of farmers don’t spray at the optimum timing—they let weeds get to 6" or taller,” says Richard Zollinger, North Dakota State University Extension weed specialist. “They’re losing the battle before they’ve even started.”

Research proves pre-emergent herbicides, when used appropriately, bring the highest return on investment. In a tilled environment, Young’s research indicates, pre-emergent, with at least two effective modes of action against target weeds and residual, runs $20 to $30 in corn and $30 to $35 in soybeans. The second mode of action might bring a higher price tag, but it helps ensure those herbicides don’t develop resistance faster. Residual helps keep weeds at bay so the crop has more time to canopy.

“I have become a firm believer in residual on my farm,” Winter says. “Keep in mind weed seeds can stay viable for 15 to 20 years.”

Weed seed dormancy and escapes mean, in some cases, pre-emergent herbicide alone might not cut it, so have a post-emergent plan. 

In post-emergent spraying, herbicide options are limited. With your contingency plan in mind, make sure you don’t repeat herbicide groups multiple years or applications in a row. Just like in pre-emergent spraying, make sure you have at least two operative modes of action against any resistant weeds.

After scouting to make sure you spray at the optimal time, Young finds post spraying in corn can cost $15 to $20 and $25 to $30 in soybeans, depending on chemistries. 

Ideally, post-emergent spraying shouldn’t be necessary if you use a good pre-emergent herbicide with residual. With effective pre-emergent herbicide, the crop will canopy, and weeds will be under control. This could save farmers $15 to $30 per acre in post-emergent application. So, if you’re going to cut herbicide costs, plan to cut in the post-emergent application, not the pre-emergent.

You will need to plan ahead for 2015-16 to make sure you have a handle on weed resistance.

“What you do in the short run can really hurt you in the long run if you don’t use a multi-year strategy and be proactive to prevent resistance from occurring [more frequently],” Shaw says. “Understand if you don’t use technology appropriately, it might be cheaper this year, but you might not have that option in three years.”

When there is a short supply of effective herbicides for problem weeds, it’s essential to plan ahead to ensure you switch modes of action and adopt a multi-year approach:

  • Consider crop rotation, climate, planting date, tillage, row spacing, weed and herbicide history to plan management practices for your farm. 
  • Analyze your weed situation. What weeds do you have? Are they resistant? What is the population? What herbicide groups are effective against them? “You cannot prevent resistance, you can only delay resistance,” Zollinger says. “If you don’t plan now, you’ll pay later.”
  • Focus on creating a sustainable herbicide program. If you rotate crops, it opens up a larger variety of herbicide choices. If you don’t rotate, you’ll need to critically consider your options to make sure you switch herbicide group year over year and still gain effective control.

“The first place I start is www.take actiononweeds.com. They show each herbicide and premix and the herbicide class,” Winter says. “I also rely on my input suppliers. I talk to them in the winter and bring in notes from the season and harvest. We use those to come up with a plan. At minimum, I look at the current growing season and two years out. When planning, I try to find out what chemistries are in the pipeline,” he adds.

New seed traits that allow chemistries to be used on alternative crops might bring more options, but they are not a silver bullet. The chemistries are still at least 30 years old and need to be treated with the same stewardship as if they did not exist.

Shaw and a team are working with USDA, the Environmental Protection Agency and other governmental agencies to reward farmers who use sustainable weed management practices.

“We are looking at these approaches so we can understand this is not a technology problem, not a biology problem but a human problem,” Shaw says. 

“Keep in mind you are working for the generations coming,” Winter says. “Take stock in the little battles. You may have 400 acres totally messed up and 200 that are good. Learn and use that. When you finally find something that works it’s an Aha! moment.”

Unfortunately, weeds don’t care about commodity prices. If you put off effective weed management because it’s costly, you will likely lose more in the long-run. Avoid a costly catastrophe and start your multi-year strategy now as you buy seed and chemicals. 

Tips for a Multi-Year Weed Control Strategy

Here are several easy ways to manage herbicide costs while still gaining adequate weed control:

  • At minimum, have a weed management plan for this season and two years out.
  • If you’re going to cut herbicide costs, cut in the post-emergent application, not the pre-emergent application.
  • Keep detailed notes of the type, height and location of the weed in each field. Record the herbicide used and its effectiveness. Use these notes when planning for next year.
  • If you don’t rotate, consider rotation to open up herbicide options and to keep the weeds guessing.
  • Consider some kind of tillage. This can cut $20 to $30 from burndown chemical costs and provides another method of control.

This story is part of a series on herbicide resistance, which puts many farmers in a “billion dollar bind.” The next two installments look at what you need to know about weeds and best practices. 


To access this spreadsheet to estimate yield and profit loss from weeds in your fields, visit www.FarmJournal.com/calculator_tools


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